Various single and dual direction pressure sensors are available utilizing a silicon diaphragm which deflects in response to pressure. Deflection of the diaphragm is generally detected by sensing elements such as piezoresistive elements placed on the edges of the diaphragm. These sensors are generally designed so that batch fabrication is possible. The range of pressure detection will depend on the size, thickness and span of the diaphragms.
To protect these sensors from hostile environments, the diaphragms are insulated by an isolator arrangement which uses an incompressible fluid to transfer applied pressure from a process environment to the sensing diaphragm. An overpressure protection device is provided to inhibit the isolator fluid from further transferring pressure to the sensor when the applied pressure reaches a preselected limit. For example, an overpressure protection device that isolatingly couples pressure to a pair of separated volumes of substantially incompressible isolator fluid is described in U.S. Pat. No. 4,949,581 issued to Stanley E. Rud, Jr. on Aug. 21, 1990. As disclosed in that patent, each volume of isolator fluid is in fluid communication with one side of the diaphragm. Pressure applied to the sensor is limited by two isolator diaphragms. When a preselected differential pressure limit is exceeded, the deflection of one of the isolator diaphragms (responding to the greater pressure) bottoms against an insulator diaphragm support. Once bottomed against the support, no further increases in pressure are transmitted to the sensor. The pressure limits of these devices are set to protect the sensor diaphragm which has a relatively low pressure limit from pressures which will permanently deform it and thereby degrade the sensor's performance.
One method that has been suggested in order to increase the range of sensor sensitivity and yet protect diaphragms used to measure lower ranges of differential pressures is to form a center boss on the diaphragms. When the diaphragm is exposed to excess pressure, the boss stops against a base and limits the deflection of the diaphragm before it is damaged. Such bosses are described in greater detail in the above referenced patent. A method of forming an overpressure stop boss extending from a diaphragm is similarly disclosed in U.S. Pat. No. 4,790,192 issued to Thomas A. Knecht et al.
A disadvantage of this design is that the deflections and thus sensitivity of the diaphragm can be affected by the overpressure boss. Diaphragms are generally less flexible in the areas of the boss and therefore likely to be less sensitive to pressure. In order to achieve the same pressure range, the diaphragm area would have to be larger than a diaphragm area without the center boss and thus some useful silicon real estate is wasted. Further, with extreme pressures, the unsupported portions of the diaphragm can rupture. Thus, the extended range of these pressure sensing diaphragms is limited.
An alternate design for a bi-directional pressure sensor, which cures some of the shortcomings of the design mentioned above, is disclosed in U.S. Pat. No. 4,905,575 issued to Knecht et al. on Mar. 6, 1990. According to the teachings of Knecht et al., a silicon diaphragm is mounted between two glass base plates which have recesses formed therein to receive the diaphragm and provide support across the diaphragm under overpressure conditions. The support plates serve as positive stops when the diaphragm is subject to overpressure and thus prevent overstressing the diaphragm. The pressure sensor disclosed in this patent further includes a diaphragm having grooves formed on opposite surfaces to define a center deflecting portion. The grooves provide a "free edge" effect which reduces bending stress at the diaphragm edge and permit a higher operating pressure without breakage.
Removing material to provide grooves on opposite surfaces of the diaphragm, however, requires tight control tolerances during manufacture. Precise alignment of the glass base supports during assembly is also critical, especially when the sensor has an array of sensing diaphragms with different sensing ranges. Further, glass and silicon differ in strength and thermal coefficients. When the sensor is intended for applications over wide temperature and pressure ranges, the material property mismatch can create stresses and large sensing errors which may be difficult to overcome.